TRYPANOSOMA EVANSI 567 



morphologically and in inoculations appeared to be T. evansi. The animals 

 were in poor condition and suffered from fever. Treatment with liquor 

 arsenicalis was carried out over a long period, during which the animals 

 were kept at work, and in two to three years they not only recovered 

 clinically, but their blood ceased to be infective to rats. Dogs are very 

 susceptible to T. evansi, and in India hunting packs have sometimes 

 suffered heavily. Death may occur in a week, or not till three or four 

 months after infection. Cats can be infected, as also pigs. In experi- 

 mental inoculations rats and mice develop very large infections, and die 

 in about a fortnight. In guinea-pigs the infection is not so intense, and 

 death results in about one month. Rabbits show still milder infections, 

 but the animals die in about the same period. Monkeys are also sus- 

 ceptible, and the disease produced terminates fatally in about two months. 

 According to Laveran (1904a), baboons (Cynocephalus) are immune. 

 Sheep and goats, though they sometimes contract a fatal infection, 

 generally recover after six months. During this period the trypanosomes 

 may be so scanty in the blood that they can only be demonstrated by 

 inoculation of blood to more susceptible rats or guinea-pigs. Goats 

 which have recovered from their infection are found to be immune to 

 reinoculation. Laveran and Mesnil have employed these immune animals 

 in the differentiation of T. evansi from other nearly allied forms. 



The virulence of a strain of T. evansi is greatly increased by successive 

 passages through small animals. In the first few passages after inoculation 

 from an infected horse the duration of life in these animals is at least 

 double what it will be later in sub-inoculations. 



Morphology.— T. evansi is a monomorphic trypanosome which always 

 possesses a fiagellum (Fig. 234 and Plate V., c, p. 456), though Bruce 

 (1911) states that rarely short stumpy forms without flagella occur. In 

 this respect it differs from T. brucei and T. gamhiense, which are definitely 

 polymorphic, in that the short stumpy forms are frequently found. 

 Measuring 820 individuals, Bruce (1911) found a variation in length of 

 T. evansi between 18 and 34 microns, with an average of 24-9. The 

 breadth is given as varying between 1-5 and 2 microns. The curve 

 (Fig. 196) shows the percentage of trypanosomes of various lengths from a 

 large number measured as compared with T. brucei. 



Transmission.— Surra is transmitted from animal to animal by various 

 blood-sucking flies, chiefly those belonging to the genus Tabanus (Fig. 211). 

 Up to the present no evidence of a cycle of development comparable with 

 that of T. (jambiense and other trypanosomes in tsetse flies has been 

 demonstrated for T. evansi. Rogers (1901) in India recorded successful 

 transmission experiments. " Horse flies " were allowed to feed partially 

 on infected dogs, and then to complete their meal on healthy dogs, 



